The use of ophthalmic lenses for the correction of ametropia is well known. For example, progressive addition lenses ("PAL's") are used for the treatment of presbyopia. On the front, or convex, surface of a PAL, three zones are provided: a distance vision zone with a refractive power suitable for distance vision; a near vision, or add power, zone, with refractive power for near vision; and an intermediate zone between the distance and near zones, with a refractive power for intermediate distance vision.
Inherent in ophthalmic lenses, such as PAL's, is an astigmatism introduced or caused by one or more of the lens surfaces, or a lens astigmatism. In PAL's, for example, the design of the front lens surface typically is constrained by the need to provide a distance zone and an add zone of maximum width connected by the intermediate zone obeying the following conditions:
f(x, y, z) is continuous; PA1 .differential.f(x, y, z)/.differential.(x, y) is continuous; and PA1 .differential..sup.2 f(x, y, z,)/.differential.(x.sup.2, y.sup.2, xy) is continuous over all x, y, z.
Such a surface inevitably introduces an astigmatism into the lens. Generally, the back, or concave, lens surface of a lens is not used to compensate for the astigmatism introduced by the front surface and, in fact, may also contribute to the lens astigmatism. Rather, the back surface typically is a spherical or toric surface intended to combine with the front surface to provide the wearer's required prescription.
Any number of lens designs have been tried to reduce lens astigmatism. However, although the known designs provide some minimal decrease in the astigmatism, large areas in the lenses' peripheries still are unusable due to the astigmatism.